1. Field
This disclosure relates to an electrolyte for a rechargeable lithium battery and a rechargeable lithium battery including the same.
2. Description of the Related Technology
Lithium rechargeable batteries using an organic electrolyte have twice the discharge voltage of conventional batteries using an alkali aqueous solution and accordingly, have high energy density.
The negative active materials of rechargeable lithium batteries have been made of various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, which can all intercalate and deintercalate lithium ions.
Composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, Li[NiCoMn]O2, and the like, have been studied as positive active materials for rechargeable lithium batteries.
One or more lithium salt dissolved in a non-aqueous solvent of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like have been used as an electrolyte for rechargeable lithium batteries.
During the initial charge of a rechargeable lithium battery, lithium ions released from the lithium-transition metal oxide, i.e., the positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as a solid electrolyte interface (SEI) film. The SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode. The SEI film prevents lithium ions from reacting with the carbon electrode and side reaction with other materials does not occur, such that the amount of lithium ions is reversibly maintained.
However, gases are generated inside a battery that uses a carbonate-based organic solvent due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction. These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. Generation of gases inside the battery causes the battery to swell in a thickness direction when it is charged. When the battery is fully charged and kept at a high temperature (for example, 100% charged at 4.2V and allowed to stand at 85° C. for four days), the organic SEI film gradually decomposes based on the electrochemical energy and heat energy that increases as time passes, continuously causing a reaction with a new adjacent surface of the negative electrode with exposed electrolyte. The continuous generation of gases increases the internal pressure inside of the battery.
There is a need for development of an electrolyte additive to suppress the volume expansion of rechargeable lithium battery. Inhibiting or decreasing gas generation during a SEI film-forming reaction is one such approach.